1. Field of the Invention
The present invention concerns the control of the columns of a matrix type LCD panel, especially a method for addressing each column of an LCD panel, and notably an active matrix LCD panel.
2. Description of the Prior Art
A matrix type LCD panel has a set of line buses and column buses which control the voltage applied t electrodes located on one and the same side of a layer of liquid crystal, the other side being occupied by a counter-electrode which cooperates with the first electrode to electrically orient the molecules of the liquid crystal and achieve the modulation of a light beam by rotation of polarization. The columns are controlled by giving each column a current for charging the capacitor set up between the column conductor and the counter-electrode in such a way that the voltage at the terminals of this capacitor represents a video signal sample between two successive addressing operations.
To obtain this charge current as a function of the video signal, it is proposed, notably when the control circuits are integrated into the LCD panel, to use a column driver transistor, the gate of which receives a control pulse as a function of the video signal sample. To illustrate this case, FIG. 1 shows the schematic diagram of a known control for an active matrix LCD panel symbolized by a column cl, a line L, a picture element P, the counter-electrode CE and a line selection thin-film transistor.
More specifically, it has been proposed, as shown in FIG. 1, to use a column control circuit comprising a voltage-duration converter 1 which receives the video signal sample E at one of its inputs and a voltage gradient coming from a gradient, generator 2 at its other input. At output, this converter delivers a pulse I, the duration t of which expresses the amplitude of the video signal sample at input. This pulse I is sent to the gate g of a driver field-effect transistor 3. One of the electrodes, or drain d in the embodiment shown, of this field-effect transistor 3 receives a voltage gradient coming from the generator 2. Its other electrode, or source S, is connected to the column bus considered. With the above-described circuit, so long as the voltage Vgs of the transistor 3 remains greater than its threshold voltage Vt, the signal on the source s follows the changes in the voltage gradient applied to the drain d, and the capacitor C, which represents the equivalent capacitance of the column, namely the liquid crystal capacitance, the unwanted capacitances of the picture element P control transistors and the intersection capacitance of the buses, gets charged. As soon as the voltage Vgs becomes smaller than the threshold voltage Vt, the transistor 3 goes off and the signal at the column keeps, as its value, the value of the voltage charged in the capacitor C. Thus, at each column cl, there is obtained a voltage which is, for example, proportional to the width of the control pulse I of the transistor 3.
In this circuit, the pulse I obtained at output of the converter 1 does not have a steep-sloped descending edge The result thereof is that the transistor 3 goes off at an instant depending on the value of the conduction threshold. Consequently, the capacitor charging voltage changes with the shifting of the threshold. Indeed, the conduction threshold gets shifted through the electrical stress undergone by the field-effect transistor 3 used to switch-over the column. This stress may be defined as the product of the gate-source voltage by the duration in which the voltage is applied. Thus, this stress is a function of the value of the input signal and, hence, of the video signal since the duration of the pulse is a function of the video signal. To illustrate this problem, FIG. 2 shows the pulse I, the voltage gradient and the column voltages V1 and V2 obtained respectively with threshold voltages T1 and T2. It is seen that the raising of the threshold voltage of the transistor tends to reduce the column voltage Vsg. Thus, it can be seen that a low value of the video signal sample generates a gate-source voltage stress which is smaller than the stress generated by a high value of the video signal sample, as shown schematically by A and B in FIG. 3. In the former case, the shift in the threshold voltage will therefore be smaller. The result thereof is a non-uniform shift in the threshold voltages of the different column switch-over transistors, leading to a non-uniformity of luminance on the LCD panel.
The present invention, therefore, is aimed at overcoming this drawback by proposing a method for the addressing of each column that prevents the switch-over threshold of the transistor from changing with the video signal samples.
The present invention is also aimed at proposing a method for the addressing of each column that enables the creation of conditions such that the gate-source stress of the transistor is, on an average, independent of the video signal sample applied to the column.